A flaw has been found in the temporary file handling algorithms for the sandboxing code used within Portage. Lockfiles created during normal Portage operation of portage could be manipulated by local users resulting in the truncation of hard linked files; causing a Denial of Service attack on the system.

Background=========

Portage is Gentoo's package management system which is responsible for installing, compiling and updating any ebuilds on the system through the Gentoo rsync tree. Under default configurations, most ebuilds run under a sandbox which prevent the build process writing to the "real" system outside the build directory - packages are installed into a temporary location and then copied over safely by Portage instead. During the process the sandbox wrapper creates lockfiles in the /tmp directory which are vulnerable to a hard-link attack.

A flaw in Portage's sandbox wrapper has been found where the temporary lockfiles are subject to a hard-link attack which allows linkable files to be overwritten to an empty file. This can be used to damage critical files on a system causing a Denial of Service, or alternatively this attack may be used to cause other security risks; for example firewall configuration data could be overwritten without notice.

The vulnerable sandbox functions have been patched to test for these new conditions: namely; for the existance of a hard-link which would be removed before the sandbox process would continue, for the existance of a world-writable lockfile in which case the sandbox would also remove it, and also for any mismatches in the UID ( anything but root ) and the GID ( anything but the group of the sandbox process ).

If the vulnerable files cannot be removed by the sandbox, then the sandbox would exit with a fatal error warning the adminstrator of the issue. The patched functions also fix any other sandbox I/O operations which do not explicitly include the mentioned lockfile.

Impact=====

Any user with write access to the /tmp directory can hard-link a file to /tmp/sandboxpids.tmp - this file would eventually be replaced with an empty one; effectively wiping out the file it was linked to as well with no prior warning. This could be used to potentially disable a vital component of the system and cause a path for other possible exploits.

This vulnerability only affects systems that have /tmp on the root partition: since symbolic link attacks are filtered, /tmp has to be on the same partition for an attack to take place.

Workaround=========

A workaround is not currently known for this issue. All users are advised to upgrade to the latest version of Portage.

Security is a primary focus of Gentoo Linux and ensuring the confidentiality and security of our users machines is of utmost importance to us. Any security concerns should be addressed to security@gentoo.org or alternatively, you may file a bug at http://bugs.gentoo.org.